The Earth’s Free Oscillations and the Differential Rotation of the Inner Core
The Earth’s Free Oscillations and the Differential Rotation of the Inner Core
Credit:
Gabi Laske, Guy Masters • IGPP, Scripps Inst. of Oceanography/IRIS Consortium
Description
a) Recent and past splitting functions for mode 13S2. The past splitting function is not accurate enough to reliably constrain inner core rotation b) Inner-core rotation rates obtained for 13 inner-core sensitive modes. Grey and symbols mark results when using our own mantle model for corrections, blue symbols mark models from other workers. Our data are most consistent with a small rotation rate of 0.24°/yr or less.
Differential rotation of the inner core (IC) was initially inferred by body-wave studies that reported a super-rotation rate of 0 to 3 degrees per year. The wide range of inferred rotation rate is caused by the sensitivity of such studies to local complexities in structure. The analysis of free oscillations is insensitive to local structure and is therefore a better candidate for estimating differential rotation.
In principle, the analysis of the time dependence of ‘’splitting functions’’ that display local frequency shifts for a mode caused by Earth’s internal 3D-structure gives us clues whether structure has changed over the last few decades. Unfortunately, the data volume and quality for earthquakes 20 years ago is insufficient to construct splitting functions with high precision. We have therefore chosen a forward approach in which we optimize the hypothesis of an assumed rotation rate against the fit of older data. We construct the ‘’recent’’ splitting function using data for recent great earthquakes. We isolate the contribution from the inner core and ‘’correct’’ the splitting function with an assumed rotation rate and determine the fit to older data. We repeat this procedure for each inner-core sensitive mode and then determine the mean inner core rotation rate. Analyzing 9 modes, our initial results indicated that IC differential rotation has essentially been zero over the last 20 years, though our error bars allowed for a small relative rotation of up to 0.3°/yr. A subsequent study with 4 additional modes and 23 additional earthquakes cut our error bars in half and our current estimate is a small super-rotation of 0.13°/yr. Since inner-core sensitive modes are also sensitive to mantle structure, we need to make sure that our result does not depend on the model we choose to correct our splitting function. It turns out that results using different models are not significantly different. However, we suspect systematic effects for certain modes that consistently give westward rotation rates. A possible process that is currently investigated is the coupling to other modes.
G. Laske, and G. Masters, The Earth’s Free Oscillations and the Differential Rotation of the Inner Core, in: V. Dehant, et al. (eds.) ‘’Earth’s Core: Dynamics, Structure, Rotation’’, Geodynamics Series 31, AGU, Washington, D.C, 5-21, 2003.
G. Laske, and G. Masters, Limits on differential rotation of the inner core from an analysis of the Earth’s free oscillations, Nature, 402, 66-68, 1999.
Photographer / Contributor: Gabi Laske, Guy Masters • IGPP, Scripps Inst. of Oceanography
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